Electro-servo system



Dec. 14, 1948. J. w. coNKLlN 2,455,979

ELECTRO SERVO SYS TEM Filed Feb. 24, 194:5

Zmventor E @film Patented Dec. 14, 1948 2,455,919 morso-Saavo SYSTEM James W. Conklin, Indianapolis, Ind., assigner to Radio Corporation of America, a corporation of Delaware Application February 24. 1943, Serial No. 478,901

(ci. 31a-3o) 101mm. l

This invention' relates to servo systems and more particularly to systems of the follow-up type for moving a driven device to a position corresponding to that of a control device. Such systems ordinarilyoperate by responding to the ditlerences between the positions of the control device and driven device, and energizing a motor to move the driven device and reduce said diilerence to zero. It has been the practice in the past to attemptto provide control means which would cause the driven device to follow the controlling device with a minimum of lag. Owing to inertia, the 'driven device tends to continue to move after deenergization ofthe driving motor, over-running theposition corresponding to that of the control member. This causes reversal of the motor control and oscillation about the point of positional agreement. Numerous more or less elaborate systemshave been devised to combat this eiiect, which is known as f'hunting, by applying braking or reverse power. Considerable deceleration forces and consequent mechanical stresses are likely to result in operation of systems of this type. In many applications, dynamic lag, provided it isl not excessive, is of no particular importance.

Accordingly it is the principal'obiect of the present invention to provide an improved method of and means for operating a servo system, so as to prevent hunting Another object is to provide an improved methodzfof -and means foroperating a servo system soas to cause the driven device to coast to rest at the equilibrium point.

It is a further object of this invention to provide improved means for operating a servo system whereby a dynamic lag is introduced and so proportioned with relation to rate of travel as to cause the driven device to Just coast to rest at a desired null point.

A further object is to provide an improved method of and means for operating a servo system so as to avoid undue mechanical stresses in the driving motor, driven device, and the connections therebetween. AThese and other and incidental objects will become apparent to those skilled in the art upon consideration of the following description with reference to the accompanying drawing, which is a schematic diagram of a servo system embodying the instant invention.

The'driven load device I is mechanically connected to an A.C. motor 3, and to the rotor of a synchro transformer 1 as indicated by the dash lines 5 and 3. The motor 3 may be of the commutatortype, provided with a separately excited field l. or may be any other conventional type of reversible lA.C. motor. The transformer 1 comprises a single phase rotor .and a polyphase stator of the .Selsyn type. Thel rotor ofthe transformer 1 isponnected to an A.C. source, not

v to resonance at tl e frequency of the A.C. supply.

cuit 33.

' operate the load device I.

2 shown. The stator of the transformer 1 is connected to the stator of a similar Selsyn" device Il, hereinafter referred to as the control transformer. The rotor of the transformer Il is mechanically connected to a control member such as a manually operable crank I3. The rotor of the control transformer Il is electrically connected to an amplifier I5 comprising electron discharge tubes I1 and I3. The amplifier I5 is coupled to a power amplifier 2i comprising-tubes 2'3 and 25. The output circuit of the amplifier 2l is coupled to the A.C. motor 3. The coupling element between the ampliiiers I5 and 2l comprises a selective filter element such as a transformer 21. tuned by .a capacitor 29 connected across its secondary A control channel 3l is provided for the amplifier I5, comprising a detectorV 33, a filter 35, and a D.C. amplifier 31. The detector 33 is coupled to the rotor of the control transformer II, and the negative potential output of the control channelfis connected to the grid return circuit of the amplifier I5. The detector 33 comprises a pair of electron discharge tubes with their grid circuits connected in push-pull and their anode circuits connected in parallel. Any conventional rectifier or detector circuit may be substituted for the cir- The filter 35 is of the low-pass type, designed to reject rectified A.C. components of the rectifier output, and to pass the relatively low frequency components corresponding to variations inthe amplitude of the output of the control transformer Il. The amplifier I5 is designed to operate as a limiter, upon the saturation principle. Resistors 39 and 4I are connected in series with the control grids I1 and I3 to limit the ow of grid currents upon positive swings oi' grid voltage, clipping the peaks of the A.C. input voltage at a level depending on the value of the negative D.C. bias voltage provided by the amplifier 31.

'I 'he operation of the above described system is as follows: v

Thecontrol device I3 is moved to a position corresponding to that to which it is desired to Assuming the position of the' load device does not then correspond to the control device, energy from the A.C. source is transmitted through the transformers 1 and II to the'input circuits of the amplifier I5 and the detector 33. The amplitude of this voltage is proportional to the difference of position between the control member I3 and the device I I, and the polaritywith resp ct to that of the A.C. supply depends upon whether the driven device leads or lags the control device. The output oi' the detector 33 is proportional in magnitude to the magnitude of the output of the control transformer II and hence, during the operation of the control device I3, to the lag vof the load device. The output of the transformer II is applied to the andere amplifier l5 and is limited thereby to a ievel dependingy on the output oi the control 'channel 8i. The ilat top -wave output oi the amplifier i@ is filtered by the'tuned transformer El, providing sine wave output. This voltage is ampliiied by the power amplifier 2i to energize the motor i3. The amplitude of the input to the amplier le increases with the angular lag, tending to increase the amplifier output and hence the energization of the motor 3. This effect occursvin the operation of the usual servo system', and is desirable in the practice of the present invention up to the point at' which the motor is just sufficiently energized to maintain enough kinetic energy stored in the load to overcome frictiqn intaking up the angular lag of the load, if the motion or the control device were suddenly stopped. At this point the control channel begins to limit the output of the amplier i5. If the speed of operation of the control device is then increased, the output of the control channel 3i will increase correspondingly, causing a decrease in the gain of the amplifier I5, and a greater angle of lag or the load device.

Thus as the driven device approaches the position of angular agreement with the control device,

the energy applied to the motor is decreased more rapidly than would bethe case if the control channel v3i were not included. With proper aciiustment of the gain of the D.C. ampliiler 31 in accordance with the dynamic friction of the load device, the motor may be caused to coast to a standstill at the point of exact positional equilibrium independently of the length or rate of operation o! the control member.

The invention has been described as an improved servo system, wherein hunting" is avoided by deenergizing the driving motor in response to the rate of approach oi the driven de. vice to positional agreement with the control means. rather than by application of braking or reverse energization. An auxiliary control channel is provided comprising a detector responsive to the difference in position of the control and driven device, and the output oi said channel is employed to control the sensitivity of the main motor control channel.

' I claim as my invention:

1. A servo system comprising a driving motor, a. load device connected to said motor, a source of alternating current, an input shaftand control means dor deriving from said source an A.C. signal related in lmagnitude and polarity to the difference in angular position between said input shaftand said device, a limiting ampliiler coupled to said control device and to said motor whereby said motor is energized in response to the output of said amplifier, and rotates-in a direction to reduce said difference, means including a rectiier for developing a D.,C. bias voltage proportional to the rate of change of said diierence, and means including said D.C. bias for reducing the limiting level of said ampliner and hence the driving torque off said motor in proportion to the rate at which the positional error between said device and said control means is reduced.

2. A device of the character described in claim 1 in which said amplifier includes grid limiting resistors for -limiting the output thereof.

3. A control system for'driving `by energy from an alternating current source a load device to an angular position corresponding to that of a control member, comprising a first synchro transformer, a second synchro transformer mechanically connected to said load device and electrically connected between said alternating current source and said iirst synchro transformer, a mechanical connection between said first synchro transformer and said control member, an ampliner having means including a series grid 'limiting 'resistor coupling the input thereof to said first synchro transformer, means for producing a negative D.C. bias voltage proportional in amplitude to the' rate of decrease of the voltage developed by said rst synchro transformer, and means for applying said bias voltage to the input of said amplifier to reduce the output thereof as a function of said rate of decrease.

4. The invention as claimed in claim 3 wherein said amplifier includes a coupling circuit tuned to resonate at the frequency of said A.C. source.

5. A servo system comprising a driving motor connected to a load device, a source of alternating current, an input shait and control means for deriving from said source an A.C. signal related in magnitude and polarity to the difference in angular position between said input shaft and said load device, an amplifier having means irlcluding a series grid limiting resistor coupling the input thereof to said ilrst synchro transformer, said motor being driven by the output of said amplifier, the value of said resistor being selected to cause the energization oi.' said motor to increase with an increase in the amplitude of said A.C. signal up to the point at which the kinetic energy stored in the load and motor is just suillcient to cause said load to coast through an angle equal to said diiierence in angular position, and means for deenergizing said motor in response to the rate of approach of said load device to positional agreement with said control means.

6. A servo system comprising a driving motor connected to a driven load device, a source of alternating current, an input shaft and positionable control means connected thereto i'or deriving from said source an A.C. signal related in magnitude and polarity to the diilerence between the position of said input shaft and the position of said load device. an amplifler for producing output currents for energizing said motor, means for applying said A.C. signal to the input of said amplifier, a grid limiting resistor included in said ampliiler for limiting the amplitude of said output currents to a predetermined maximum value, and means responsive to the rate of decrease of amplitude of said A.C. signal for deenergizin said motor.

l 7. A device of the character described in claim 6 in which said last namedr means comprises means for producing a D.C. voltage whose amplitudes is proportional to the amplitude of said A.C. signal. means for deriving therefrom a D.C. bias voltage proportional to the rate of change ot amplitude thereof, and means for applying said bias voltage to said amplifier for reducing the amplitude of said output currents.

JAMES W. CONKIJN.

REFERENCES CITED The following references are of record in the nie of this patent:

UNITED STATES PATENTS Date 

